16 PEOCEEDINGS OF THE NATIONAL MUSEUM. vol. 66. 



altered zone along a parting parallel to (010). The pyroxene has 

 quite clearly crystallized before the other constituents of the rock 

 and probably was subjected to some deforming stresses before final 

 consolidation, for many of the blades are bent considerably. The 

 diallagic parting, which is considered to be due to pressure, was 

 developed before the final consolidation of the magma, because many 

 of the larger pyroxenes are seen to have separated along this parting 

 into leaves between which are layers of feldspathic ground mass. 



The other constituents of the rock are plagioclase and micropegma- 

 tite with accessory iron ore, biotite, and chalcopyrite, all of which 

 are, at times, visible under a lens. 



Under the microscope the rock is like the normal diabase in com- 

 position except for the greater abundance of micropegmatite (pi. 4, 

 lower). The pyroxene has the same nonpleochroic pale violet-brown 

 color and shows the t winnings and partings previously described. 

 The greater portion of the large blades are fresh, clear, and trans- 

 parent, but there are patches where abundant minute dustlike inclu- 

 sions are developed along the basal parting. Other areas sometimes 

 show more intense alteration and are then filled with grains of iron 

 ore and minute shreds of biotite, chlorite, and hornblende. Some 

 partly altered blades show an outer border, in parallel position, of a 

 hornblende pleochroic in tones of light brownish olive green and deep 

 olive green. 



The feldspar occurs as rather large crystals of plagioclase which 

 shade outward into surrounding haloes of micropegmatite. The 

 plagioclase is a labradorite of rather uniform composition, the /S index 

 of refraction being about 1.560 indicating Ab^g Augj. A section 

 showing both albite and carlsbad twinning yielded extinctions indi- 

 cating Ab^o An go. The albite twinning lamellae are narrow and 

 rather sparsely distributed. Pericline twinning is also frequently 

 developed. There is no extensive alteration of the feldspars, although 

 rather large irregular patches of shreds and flakes of a micaceous min- 

 eral of high birefringence are occasionally developed in them. 



The micropegmatite is a prominent constituent of the rock and 

 consists of a beautiful pattern of quartz and feldspar. There are 

 two kinds, orthoclase micropegmatite and plagioclase micropegmatite. 

 The plagioclase micropegmatite surrounds the plagioclase crystals 

 and the feldspar of the central crystal is optically continuous with 

 that of the surrounding micropegmatite, albite twinning lamellae 

 being traceable out into the micropegmatite while crooked fingers 

 of quartz penetrate the central feldspar crystal. The index of the 

 feldspar in the plagioclase micropegmatite is well above that of the 

 balsam of the slide, and the feldspar is limpid and free from altera- 

 tion. Quartz-orthoclase micropegmatite is common interstitially 



